feat(typescript): Add support explicit null values

[amckinney]

Now that the Fern definition supports nullable properties (re: #5601), this upgrades the TypeScript generator to IRv55 to support this feature. For example, consider the following Fern definition:

types:
  User:
    properties:
      name: string
      tags: nullable<list<string>>
      metadata: optional<nullable<Metadata>>

  Metadata:
    properties:
      createdAt: datetime
      updatedAt: datetime
      avatar: nullable<string>
      activated: optional<nullable<boolean>>

The generated TypeScript types look like the following:

export interface User {
    name: string;
    tags: string[] | null;
    metadata?: Metadata | null;
}

export interface Metadata {
    createdAt: Date;
    updatedAt: Date;
    avatar: string | null;
    activated?: boolean | null;
}

Note that this also temporarily disables the dynamic snippet tests so that we can more confidently test the TypeScript SDK generator until example generation is fixed.

[Swimburger]

optionalType => nullableType

[Swimburger]

optType => nullableType

[Swimburger]

unrelated, but are we creating a copy of an array here? I think there are simpler ways to do this.

[github-actions]

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